Centralized communication system



May 26, 1959 D. A. COHN CENTRALIZED COMMUNICATION SYSTEM Filed May 12, 1958 wiwisfg H 0M0 A. (MN

M98 #386 w w /Q M 2 w QM N J vN 335% v kGmQQQR mw United States Patent CENTRALIZED COMMUNICATION SYSTEM David A. Cohn, Pearl River, N.Y., assignor to David Bogen (Zompany, Division of The Siegler Corporation, Paramus, NJ a corporation of Delaware Application May 12, 1958, Serial No. 734,738 7 Claims. (Cl. 179-2 provides communication and signalling facilities to a num-' ber of separate locations from a master or central control point. Systems of this type are often used in schools and other institutions for the distribution of both program material and time signals. Program material may comprise music, lectures, announcements, and the like, which are transmitted from the master control point to one or more of the rooms in the institution. Time signals are usually transmitted from the control point to one or more of the rooms to signal the beginning and end of certain periods, such as class periods in schools, for example. Provision may also be made-for the distribution of special signals, such as fire or civil defense warning signals, as well as for inter-room communication.

- One of the basic problems associated with centralized communication systems is that of cross talk between the program signals and the time signals in the loudspeakers.

of the system. If a single loudspeaker is employed in each room to emit both program material and time signals and the loudspeakers are coupled to the sources ofprogram and time signals in parallel circuit arrangements, it is apparent that the application of a time signal to even a single room will result in spurious time signals being applied to the other rooms, due to the well known cross talk efiect. introduction of spurious time signals, it is with respect to these signals that the problem is most serious. This follows because program signals are usually of a nature which permits their being transmitted simultaneously to a number of the rooms in the institution, while time signals are generally transmitted in accordance with a predetermined schedule that is arranged specially for each room. For example, it may be desired that a musical program be transmitted to each room of the institution during the luncheon recess, yet a time signal indicat-v ing the start of a class period or work period be transmitted to only a limited number of rooms where such,

activities are to begin. Means are usually provided to discontinue the transmission of the musical program to those rooms where the time signal has been sounded, however, the time signals themselves may be introduced as spurious time signals in the remaining rooms and cause confusion among the occupants.

, A number of arrangements for reducing cross talk in centralized communication systems have been proposed in the past. One such arrangementcontemplates the use of separate loudspeakers in each room for the program and time signals. Even if a coaxial type of loudspeaker could be substituted for the two separate loudspeakers, it is apparent that this arrangement is quite expensive and space-consuming. Another proposed arrangement employs relays to discontinue the application of program signals to a loudspeaker when time signals are applied. This ofcourse requires that the normal While cross talk is not limited solely to the,

2,888,518 Patented M program signal connections be broken, so that simul'-' taneous emission of program and time signals from the same loudspeaker is not possible. Still other arrangements proposed for reducing cross talk require the use of balancing circuits or attenuating networks, both of which suifer from the defect that they attenuate the transmitted program signal as well as the undesired cross talki Accordingly, it is an object of this invention to provide a method and apparatus for reducing cross talk in cen-' tralized communication systems without incurring the above disadvantages inherent in the prior arrangements.

Briefly, the present invention contemplates the use of a single loudspeaker in each of the locations served by the centralized communication system. Each loud speaker has a voice coil which is divided into a number of sections equal to the number of signal sources in the system. When only program signals and time signals;

are to be transmitted, the voice coil is divided into two sections. One section of the voice coil of each loudspeaker is coupled to the source of time signals by selec tively operable coupling means which permit individual scheduling of the time signals applied to each room. The

other section of each voice coil is coupled to the source of program signals through an amplifier, so that these sections are all in parallel circuit with respect to each other across the apparent output impedance of the ampli- Finally, means are provided to reduce the apparent output impedance of the amplifier, whereby cross talk' resulting from time signals in one loudspeaker being coupled to other loudspeakers through the apparent output impedance of the amplifier is substantially elimi-" nated.

The invention will now be described in detail by reference to the single figures of the accompanying drawing, which shows a schematic circuit diagram of a centralized communication system embodying the teachings of the invention. i

As seen in the drawing, loudspeakers 10A, 10B, and 10Z are mounted in corresponding rooms A, B and Z' of an institution serviced by the communication system. Loudspeaker 10A has a voice coil divided into sections 11A and 12A, so that energization of either or both sections of the coil drives the loudspeaker. Section 11A of the voice coil is connected across the secondary winding 13A of a conventional coupling transformer 14A, whilesection 12A is similarly coupled across the secondary winding 15A of another coupling transformer 16A. By this means, each section of the voice coil is adapted to be independently energized through separate coupling transformers. A primary winding 17A of transformer 14A is connected by leads 18A and 19A to one side of a double pole-single throw switch 20A. The 'other side of switch 20A is coupled by leads 23 and 24 to the out put of a power amplifier 25 which is supplied by program signals from a source 26. Source 26 may comprise any source of program material that is required by the insti tution served by the centralized system. For example, it

may include a microphone for making announcements or giving lectures and a playback device for tapes or records. The power amplifier 25 may be any conventional amplifier that is capable of amplifying the program signals from source 26 with the gain required by the overall system. An additional requirement imposed o amplifier 25, for reasons which will be set forth herein- I the output of amplifier 25 in a parallel-circuit arrangement. Since the same coupling arrangement is used for the loudspeakers 10B and NZ, as is used for loudspeaker 10A, a detailed description thereof is believed unnecessary. The system, as thus far described, permits each of the loudspeakers to be energized by program signals from the program signal source 26. Switches 20A, 29B and 292 are included in the coupling between each loudspeaker and the program signal source, so that the application of program signals to any or all of the rooms may be discontinued if desired.

Provision is also made in the system for the distribution of time signals to any or all of the rooms on an individual schedule for each room. To this end, the primary winding 27A of coupling transformer 16A is coupled by leads 28A and 29A to a time schedule crossconnecting panel 30. The panel 30 is provided with a series of different time schedule signals 1, 2 and N from a switching assembly 49. The switching assembly receives a basic time signal tone from the output of a signal power amplifier 58, which, in turn, receives signals from an oscillator 68. Both amplifier t and oscillator 60 may comprise any conventional type. The switching assembly 49 includes relay contacts 41, 42 and 43, each of which has two positions. Contact 41 is located in the lead supplying time schedule signal l to the crossconnecting panel 3%), while contacts 42 and d3 are similarly located in the leads supplying time schedule signals 2 and N to the panel. As seen in the drawing, the upper position of each contact connects the constant time signal from oscillator 68 to the leads supplying panel 30 with the time schedule signals. The lower position of each contact disconnects the constant time signal from the panel, so that each contact is adapted to control the application of a particular time schedule signal. A clock controller '76 which may be of any suitable type, such as an IBM program clock, for example, delivers spaced pulses to relay coils 44, 45 and 46, which are located in the switching assembly as. A lead 47 provides a common return for the relay coils, so that each coil may be separately energized. By the arrangement thus far described, any schedule of time signals that may be desired can be applied to the cross-connecting panel 30 by suitably programming the clock controller 70. Similarly, by making the proper connections in the cross-connecting panel, any one of the loudspeakers A, 10B and 102 may be coupled to receive any schedule of time signals desired. It may be noted that when primary windings 27A, 27B and ZIZ are not coupled to receive time signals, they are short-circuited by the action of the respective relay contacts 41, 42 and 43, so that no direct coupling exists between them.

The cross talk problem which arises in a system of this type may be readily appreciated from the following example. Assuming that switches ZiiA, 20B and ZOZ are all closed and a program signal, such as an announcement, is being distributed to each of the loudspeakers from source 26, it is apparent that the application of a time signal to loudspeaker 10A will produce spurious time signals or cross talk in loudspeakers 16B and 102. This follows from certain characteristics of the coupling circuits for each of the loudspeaker voice coils. If the spurious time signals in loudspeakers MB and NZ are applied simultaneously with the announcement signal, the cross talk may not be noticed, especially if the signal level of the announcement is high. However, should the spurious time signal be applied to these loudspeakers during a quiet period in the announcement, they would be very noticeable, especially if the ambient noise level in the rooms involved is low. The characteristics of the coupling circuits for the loudspeaker voice coils which affect cross talk comprise the mutual inductance between the two sections of each voice coil and the leakage inductance or the coupling transformers. By the use of well known design techniques and a careful selection of component parts, it is possible to make a substantial reduction in cross talk of the order of 12 to 20 db. However, a reduction in cross talk of this magnitude would probably be inadequate under conditions of low ambient noise in the rooms and non-simultaneous reception of program and time signals.

This invention recognizes that the apparent output impedance of the program power amplifier 25 may be altered to reduce or even completely eliminate cross talk. It may be noted that the primary windings 17A, 17B and 172 of the coupling transformers are all connected in parallel circuit across the output of amplifier 25. Therefore, the apparent output impedance of the amplifier, i.e. the impedance as viewed from the load, acts as a common coupling impedance with respect to the primary windings which carry the program signals. Since the time signals applied to a particular loudspeaker are reflected in the primary winding of the coupling transformer carrying the program signals, due to the aforementioned characteristics of the voice coil coupling circuits, it is seen that the primary windings act as generators of the spurious time signals. Accordingly, by substantially reducing the apparent output impedance of the amplifier, it is possible to substantially reduce, or even eliminate, cross talk in the other loudspeakers. For example, if the apparent output impedance of the amplifier is equal to the composite load, as presented by the primary windings, a further reduction of 6 db will be obtained in cross talk. Similarly, a reduction of 20 db can be obtained by making the apparent output impedance of the amplifier equal to one-tenth of the comp Jsite load. Ultimately, cross talk may be completely eliminated by reducing the amplifier output impedance to zero.

A number of methods are known for reducing or altering the apparent output impedance of an amplifier by the use of feedback. These methods would be suitable for the purposes of the present invention. For example, it is known that the apparent output impedance of an amplifier may be reduced by the application of negative voltage feedback signals to the input of the amplifier. A discussion of this method, as well as a disclosure of circuits for performing it, may be found in sections 5-7 and 5-8, beginning on page 85, of Electron-Tube Circuits by Seely, McGraw-Hill Book Company, Inc., 1950. Another method of reducing the apparent output impedance of an amplifier may be found in copending patent application, Serial No. 606,874, by Charles A. Wilkins. This latter method makes use of compound feedback and applies both negative voltage feedback signals and current feedback signals to the input of the amplifier. Suitable circuitry for performing this method may also be found in the aforementioned patent application. It may be pointed out that the use of the feedback circuit disclosed in the Wilkins application permits the apparent output impedance of an amplifier, such as amplifier 25, to be reduced to zero, thereby completely eliminating cross talk.

As many changes could be made in the above construction and many apparently widely dilferent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a centralized communication system for selectively transmitting signals from at least two signal sources to a plurality of locations, the combination comprising a plurality of loudspeakers, each of said loudspeakers being associated with a different one of said locations and having a voice coil divided into at least two sections; selectively operable coupling means for coupling one section of the voice coil of each of said loudspeakers to one of said signal sources, so that said one section of the. voice coil of each of said loudspeakers is adapted to be energized by the signals from said one signal source; an amplifier having an input and an output; means for coupling the input of said amplifier to the other of said signal sources; means for coupling the output of said amplifier to the other section of the voice coil of each of said loudspeakers, so that said other sections of the voice coils of the loudspeakers are in parallel circuit with respect to each other across the apparent output impedance of said amplifier and are adapted to be energized by the signals from said other signal source; and means for substantially reducing the apparent output impedance of said amplifier, whereby cross talk between the signals from said signal sources in substantially eliminated in each of said loudspeakers.

2. Apparatus as claimed in claim 1, wherein said lastnamed means comprises feedback means for feeding back a portion of the output signals of said amplifier to the input of the amplifier.

3. Apparatus as claimed in claim 2, wherein said feedback means applies negative voltage feedback signals to the input of said amplifier.

4. Apparatus as claimed in claim 2, wherein said feedback means applies both negative voltage feedback signals and current feedback signals to the input of said amplifier.

5. In a centralized communication system for selectively transmitting signals from a source of program signals and a source of time signals to a plurality of school rooms and the like, the combination comprising a plurality of loudspeakers, each of said loudspeakers being located in a difierent one of said rooms and having a voice coil divided into two sections; a pair of coupling transformers for each of said loudspeakers, each of said coupling transformers having a primary winding and a secondary winding; means for coupling the secondary windings of each pair of transformers to the sections of and an output; means for coupling the input of said amplifier to said source of program signals; means for coupling the output of said amplifier to the primary winding of the other transformer of each pair of transformers, so that the primary windings of the other transformers of the pairs of transformers are in parallel circuit with respect to each other across the apparent output impedance of said amplifier and each of said loudspeakers is adapted to be energized by program signals; and means for substantially reducing the apparent output impedance of said amplifier, whereby cross talk resulting from time signals in one loudspeaker being coupled to other loudspeakers through said apparent output impedance is substantially eliminated.

6. Apparatus as claimed in claim 5, wherein said lastnamed means comprises feedback means associated with said amplifier for applying negative voltage feedback signals to the input of the amplifier.

7. Apparatus as claimed in claim 5, wherein said lastnamed means comprises compound feedback means associated with said amplifier for applying both negative voltage feedback signals and current feedback signals to the input of the amplifier.

No references cited. 

